Infectious diseases are a leading cause of death world-wide and there is a growing need for new anti-infective agents to combat multi-resistant strains of bacteria and fungi. In the present thesis, an investigation into the antimicrobial metabolites of Arctic and sub-Arctic marine invertebrate species is presented. Extracts of sponge, ascidian, coral and bryozoan species were pre-fractionated by solid phase extraction (SPE) and purified by reverse-phase high-performance liquid chromatography (RP-HPLC). Active metabolites were characterized by electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) techniques. The work resulted in the isolation and characterization of a number of new metabolites including the novel synoxazolidinones A, B and C, which exhibited antimicrobial and anticancer activities. In addition, the thesis provides background information on natural product research and current antimicrobial investigations of marine invertebrate species. This work shows the potential of Arctic and sub-Arctic marine invertebrates as sources of structurally novel, bioactive metabolites.